The present invention relates to the measurement of environmental icing conditions, and more particularly, to a method for improving the measurement accuracy of a magnetostrictive oscillator ice rate sensor probe.
Throughout aviation history, icing conditions have been an issue for aircraft. It is a vital part of flight safety to inform the flight crew of ice or icing conditions. The formation of ice as a result of super-cooled liquid water content (LWC) accreting on aircraft surfaces can take place anywhere including engines, air induction systems, and control surfaces. Determining when ice is starting to form or predicting when it will form is important in aircraft management of deicing equipment including heaters, which can consume huge amounts of power. The accurate and timely measurement of liquid water content (LWC) permits prompt signaling for alerting the flight crew, activating deicing systems, and for data gathering and reporting.
As technology has advanced, improved methods of ice detection have been developed, with a variety of different technologies that have been or currently are deployed. A current method that is employed to measure LWC utilizes a magnetostrictive oscillator (MRO) vibrating probe that extends into the airstream, with a driving circuit that senses the resonant frequency of the probe. For example, Severson, et. al., U.S. Pat. No. 6,560,551 discloses a vibrating probe type ice detector. The ice growth rate is predictably variable over an accretion cycle based upon the incremental rate of change of the vibrating probe's frequency throughout the sensing cycle. The time rate of change of resonant frequency df/dt throughout the ice accretion cycle is determined. As ice accretes on the probe, the probe's resonant frequency decreases due to the increase in mass. At the end of the ice accretion cycle a heater internal to probe is energized to rapidly melt the ice on the surface on the probe. As the ice melts from the probe, the resonant frequency rapidly increases back to the nominal resonant frequency. The heater is deenergized, the probe surface rapidly returns to an equilibrium condition with the ambient conditions, and the next ice accretion cycle begins.
The physical dimensions and mass of the probe may vary widely from one design to another, with these values contributing to the nominal resonant frequency of a particular design. For example, the Rosemount Icing Detector Model 871 is designed to have a nominal resonant frequency of 40.0 KHz. This exemplary design is one of many that are available in the aviation industry. In this exemplary ice detector, an ice accretion cycle begins with the resonant frequency at 40.0 KHz and the frequency decreases as ice accumulates on the ice detector probe. The time rate of change of resonant frequency df/dt is measured during the ice accretion cycle, with the cycle ending when the resonant frequency decreases to 39.7 KHz. At the end of this cycle, the probe heater is energized to melt the accumulated ice. Under some conditions, the accretion cycle will be ended prior to reaching this ending resonant frequency. The period of the accretion cycle will vary widely, and will depend on multiple factors including the ambient temperature, density, pressure, LWC, and airspeed. The accretion cycle period may range from a few seconds through several minutes, or longer.
A problem with MRO ice rate sensor systems is that their system accuracy may often worse than 20%, which may be considered unsatisfactory for certain applications. Knowing with accuracy when icing conditions exist, and therefore accurately measuring the liquid water content of the airstream near the aircraft during flight operation, is important because of the adverse effect that icing has on aircraft performance and safety. Several attempts to improve the measurement accuracy of vibrating probe ice rate sensors have been deployed. For example, Otto, et. al., U.S. Pat. No. 7,104,502 discloses a probe support strut containing one or more features which allow the probe to accrete ice at a higher temperature than would conventionally be possible.
Despite many improvements that have been advanced in this field, there is an ongoing need to improve the overall accuracy of the MRO digital ice rate sensor. A design target for an MRO digital ice rate sensor is to measure the LWC in situ with an accuracy of 20%. Achieving this target will enable an aircraft flight crew to operate within a flight envelope that minimizes the adverse effects of icing, some of which have been described above.